This invention generally relates to electron beam generators, particularly of the type used in electron beam physical vapor deposition apparatuses to deposit ceramic coatings. More particularly, this invention is directed to an electron beam generator that exhibits improved service life at high operating temperatures.
Electron beam physical vapor deposition (EBPVD) is a well-known process for producing ceramic coatings, such as thermal barrier coatings (TBC) for the high-temperature components of gas turbine engines. Various ceramic materials have been used as TBC""s, particularly zirconia (ZrO2) stabilized by yttria (Y2O3), magnesia (MgO) or other oxides. Advantageously, TBC""s can be deposited by EBPVD to have a columnar grain structure that is able to expand with its underlying substrate without causing damaging stresses that lead to spallation, and therefore exhibits enhanced strain tolerance.
Processes for producing TBC by EBPVD generally entail heating a component to be coated to a temperature of about 1000xc2x0 C. or more within a partially evacuated coating chamber. During coating, the component is supported above an ingot of the ceramic coating material (e.g., YSZ), and an electron beam generated by an electron beam (EB) gun is projected onto the ingot to melt the surface of the ingot and produce a vapor of the coating material. The vapor then travels upward toward the component and condenses on the component surface to form the desired coating. In order to melt ceramic materials such as YSZ, electron beam guns must be operated at a high voltage (e.g., 35 kV) and power level (e.g., 50 to 120 kW). The EB gun component that produces the electron beam is a beam generator. FIG. 1 represents a generator 110 that is commercially available from ALD Vacuum Technologies, Inc., of East Windsor Conn., USA. The generator 110 has a primary cathode (filament) 140 which produces an electron flux that heats a primary tungsten anode (block) 148 to about 2000xc2x0 C. The block 148 then serves as a secondary cathode to an external secondary anode (not shown), by which the tungsten block 148 emits an electron beam due to a high voltage between it and the secondary anode. If any connection in the circuit containing the filament 140 becomes resistive due to oxidation, or mechanically opens due to thermal stress, or both, the beam generator 110 ceases to emit, stopping evaporation and terminating the coating process.
The filament circuit contains several bimetallic contacts in close proximity to the hottest section of the generator 110. The two metals most widely used are copper and molybdenum, the former for its electrical and thermal conductivity and the latter for its high melting point and stability at high temperatures. For example, a conductor rod 112 that delivers current to the filament 140 is most often copper. A molybdenum ion catcher 128 has a first end 130 threaded into a bore 126 of the conductor rod 112, by which a molybdenum spacer 124 is secured to the rod 112. A first molybdenum filament tower 138 is then secured and electrically connected to the spacer 124 with a threaded stud 160 and copper nut 162, which clamps a disk-shaped insulator 166 between the spacer 124 and tower 138. A second molybdenum filament tower 139 is secured with a second stud and nut assembly 160/162 to the insulator 166, between which is clamped a molybdenum mounting plate 164. As such, both of the filament towers 138 and 139 are secured in place as a result of the spacer 124 being secured to the rod 112 with the ion catcher 128. The rod 112, spacer 124, ion catcher 128 and filament tower 138 constitute a forward leg of the filament circuit. Because of their high temperature environment, the threaded connections can loosen and oxidize during operation due to differing expansion and heat conduction of the two metals. If the ion catcher 128 becomes loose and releases the spacer 124, the filament circuit opens and the generator 110 cannot be restarted.
The filament tower 139, a molybdenum cap 144, the molybdenum mounting plate 164, a copper fitting 142 and a copper guide tube 134 constitute the return leg of the filament circuit. The molybdenum cap 144 is threaded onto the copper fitting 142, which is brazed or otherwise permanently attached to the copper guide tube 134 surrounding the conductor rod 112. The cap 144 clamps the mounting plate 164 to the fitting 142 to complete the filament circuit between the tower 139 and the guide tube 134. Consequently, if the cap 144 loosens, the filament circuit is open and the generator 110 ceases operating. The threads of the fitting 142 can distort at the high operating temperatures of the generator 110. In addition, the threaded portion of the cap 144 may bell and crack during extended operation of the generator 110. If the clamping action between the cap 144 and fitting 142 is lost, the filament circuit opens, again with the result that the generator 110 shuts down and cannot be restarted.
In view of the above, it can be appreciated that improved service life for an EB gun could be obtained if the reliability of the EB generator 110 mechanical connections could be improved. However, any change in the mechanical design of the generator 110 must be made carefully and tested with caution due to the very high operating voltages, power levels, amperage and operating temperatures involved.
The present invention is an electron beam generator of the type used in an EBPVD apparatus. A feature of the generator is that critical interconnections between individual components are made less prone to the adverse effects of thermal cycling.
The generator of this invention generally includes a conductor rod within a guide tube, generally as done in the prior art. However, the adjacent ends of both the conductor rod and guide tube are configured differently for purposes of the invention. The end of the conductor rod is modified to accept one end of a center conductor member. The opposite end of the center conductor member is formed to have an integrally-formed flange extending radially therefrom. An outer conductor member is secured to the adjacent end of the guide tube. A first tower is secured and electrically connected to the flange of the center conductor member, while a second tower is adjacent the first tower and electrically connected to the outer conductor member. A filament is mounted to and between the first and second towers, and a member is positioned adjacent to the filament for generating an electron beam when a sufficient current is applied to the filament via the conductor rod, the center conductor member, the flange and the first tower, which constitute a forward leg of the filament circuit. A return leg of the filament circuit includes the second tower and the guide tube, interconnected by the outer conductor member.
An important feature of the invention is the elimination of the discrete spacer 124 between the ion catcher 128 and the conductor rod 112 of the prior art generator 110 of FIG. 1. Instead, the function of the spacer 124 is performed by the integral flange of the center conductor member, which serves as an intermediate connector between the first tower and the conductor rod. By eliminating the need for a discrete spacer and therefore the possibility of it loosening, the center conductor member is able to considerably reduce the possibility of an open circuit occurring between the ion catcher and the conductor rod as compared to the prior art generator 110.
According to one aspect of the invention, at least one and preferably each of the center conductor member, first and second towers, outer conductor member and cap are formed of stainless steel, instead of the conventional molybdenum. As a result, the differences in coefficient of thermal expansion are less between the stainless steel components and the conventional copper components, as compared to that between the conventional molybdenum and copper components of the prior art.
According to another aspect of the invention, the second tower is preferably secured and electrically connected to the outer conductor member by a mounting member, which in turn is clamped to the outer conductor member. The mounting member is preferably clamped to the outer conductor member with a cap that is secured to the outer conductor member by a camming feature, instead of being secured with threads directly to the guide tube. As a result, another benefit of the invention is the reduced likelihood of an open filament circuit occurring as a result of thread distortion between the cap 144 and fitting 142 of the prior art generator 110 of FIG. 1.
Other objects and advantages of this invention will be better appreciated from the following detailed description.